In order to promote adhesion between rubber and ferrous metals it is known to employ a variety of metallic salts as coatings to the metal or as an ingredient in a rubber composition. It is also known to add various resins as tackifiers and/or adhesion promoters and, in other instances, to employ both a metal salt and a resin. Typical of the first type of art is U.S. Pat. No. 2,912,355 which is directed toward improving the adhesion between rubber and metal by the incorporation into a rubber composition of a calcined, partially oxidized metal salt of an aliphatic fatty acid compound, the metal being cobalt, copper, iron, lead, mercury, nickel or silver.
The use of resins alone in rubber compounds is disclosed in U.S. Pat. No. 4,146,513 which provides a modified phenolic resin which is employed as a tackifier in natural/synthetic rubber stocks where natural building tack has been decreased due to the presence of the synthetic rubber. The patent also reports improved static adhesion between the rubber and reinforcement filaments, such as brass-coated steel wire, when the tackifier resin of the invention is employed.
Finally, patents which disclose the use of both metal salts and resins include U.S. Pat. No. 3,897,583 which is directed toward the adhesion of metal to rubber by incorporating a cobalt salt in a rubber stock which contains an adhesive resin forming system based on methylene donor which is a methylolated nitroalkane in combination with a resorcinol type methylene acceptor. Cobalt salts disclosed include those of aliphatic or alicyclic carboxylic acids having 6-30 carbon atoms.
U.S. Pat. No. 4,076,668 discloses the use of cobalt salts of organic acids, the latter having between 6 and 30 carbon atoms with a monohydroxybenzoic acid component in natural and/or synthetic rubber in order to improve the adhesion between the rubber composition and a variety of reinforcements including plated steel cord. The patent further discloses that the adhesive properties of the rubber composition with fibrous materials, e.g., nylon, Aramid and the like, can be improved along with moisture resistance by the addition of an alkylphenol type resin or cresolformaldehyde type resin.
U.S. Pat. No. 4,148,769 is directed toward a sulfur-containing rubber composition having improved adhesion with brass-plated steel cords comprising, as unconventional ingredients, litharge and a cobalt salt of a fatty acid. The patentees further state that adhesive strength is improved by the addition of resorcinol, or a prereacted soluble resorcinol resin. Resorcinol resin has long been known to increase the adhesion between rubber and brass. The data in the patent indicates that even more satisfactory results are obtained when the cobalt salt, litharge and resorcinol are incorporated together in the rubber composition.
A final U.S. Pat. No. 4,258,770, owned by the common Assignee, is directed toward the use of certain inorganic salts of cobalt or nickel in combination with a rosin-derived resin as additives to a vulcanizable rubber. Improved rubber to metal adhesion and adhesion retention between the rubber and plated steel cord is reported.
A 1967 Czechoslovakian Pat. No. 125,228 discloses closes the addition of cobalt propionate as an adhesive for improving the adhesion between unplated metal and synthetic or natural rubber stocks. The composition of the rubber stock also includes a quantity of pine tar.
United Kingdom application No. 2,060,650, filed by Pirelli and published May 7, 1981 discloses a method for bonding elastomers to ferrous metal, particularly brass-plated steel. The method is practiced by adding a quantity of nickel abietate which is described as the salt of nickel and colophony, the latter consisting of 98 percent abietic acid. The elastomeric stocks tested also contained quantities of resorcinol and hexamethylene tetramine as bonding agents and thus, no adhesion values were presented for the nickel/colophony salt addition alone without the two aforementioned bonding agents.
While others have sought to enhance adhesion between rubber compositions and metals by employing various combinations of cobalt and other metal salts with resins, the art of which has been presented herein has not disclosed the exclusive use of an organic salt of nickel with a thermoplastic resin derived from crude wood rosin to increase adhesion properties between rubber and brass-plated metallic reinforcement.